CRT implosion screens.
 

This could be an interesting topic.

I noticed in early American TV's the CRT face was protected by a twin layer of laminated glass (RCA 621TS for example). Though some sets used an acrylic like plastic, that would go hazy with age.

Later on the CRT's, both monochrome & color were made with much thicker faceplates and had integral implosion protection and didn't require a cover of any kind. The face was exposed to the user to clean (or scratch) etc.

A number of other CRT's had a "Bonded Faceplate" with a sheet of tough glass that matched the curvature of the CRT's faceplate glued on with an adhesive.

The original CRT in my Conrac monitor was a green phosphor type. It had a glue bonded implosion screen (I have described this monitor an another thread). When I replaced this CRT with a P4 (white) phosphor type, I used a type without a bonded faceplate and it made me wonder, because the face was then exposed to knocks & scratches.

This is something to give thought to with any project using a CRT where it might have a vulnerable faceplate.

So I decided to organize an implosion screen from 5mm acrylic. The first attempt was laser cut out of transparent (non tinted) material. I didn't like it much because the laser cut edge had microscopic ridges. I wanted a clear light blue, but I couldn't get the material here, I had to order it from the UK.

The pale blue tinted material was then cut the old fashioned way mechanically with a router and the edges polished with polishing compound at the local plastics company, giving a much better finish than the laser cut. Pictures attached. So I think the CRT faceplate is better protected now.

Photos attached of the two attempts at the implosion screen, clear & pale blue. It simply mounts under the existing front panel handles on the monitor.It includes 4 extra holes so the 4 CRT release screws still operate.

(This monitor was worth the work, also shown are images it produces with outstanding brightness, contrast, focus, resolution & scanning linearity, plus this unit has the best black level clamp on the planet).

Re: CRT implosion screens.
 

An old pot from Darius about his CRM92:

https://www.vintage-radio.net/forum/...ead.php?t=5505

Jac

Re: CRT implosion screens.
 

By circa 1960, dad had accumulated a collection of perspex and glass implosion screens, passed on from our neighbour, a retired rag and bone man who broke up old TVs and radios for their copper content. I well remember dad trying to cut one down to size to replace a pane of glass for his shed: he had a diamond-tipped glass cutter that usually worked OK. The first turned out to be laminated which broke OK on the scribed side but cracked all over the place on the unscribed side. The second turned out to be armour plate that scribed OK but defied all attempts to make it break, even when in frustration he hit it as hard as he could. He ended up going to the glazier's. I seem to remember pink, grey, and clear perspex.

Re: CRT implosion screens.
 

I can remember trying to recycle a sheet of glass from an old oven door.
When I tried to cut it the thing exploded leaving me a mess to clear up.

Re: CRT implosion screens.
 

I once saw a pressed glass hors d'oeuvre dish spontaneously explode. There was nothing hot in it.

Re: CRT implosion screens.
 

Pyrex in mysterious stuff and does explode for the most odd reasons.
I once dropped a Pyrex tea cup onto a tiled floor and it bounced three times and when I was about to catch it after the fourth bounce it exploded.

Re: CRT implosion screens.
 

TV tube safety was my job. Tested thousands. Used to swing a glass ball at them and scratch the screen then squirt with liquid nitrogen...

In all the thousands I tested only about 5 'blew' and when they did it was really severe and usually due to a manufacturing fault. The whole building shook, glass was atomized and turned into fine shards and dust the stuck in the walls and got everywhere. Shadow mask hanging out, room filled with dust. Horrible. After seeing that happen I never saw TV tubes in the same way - I knew what they were capable of.

Most were VERY safe, just the screen holed or neck dropped off. Older screens may well not have all the safety features.

Re: CRT implosion screens.
 

Several posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=145785

Re: CRT implosion screens.
 

Wow. I guess if one considers that the CRT has excluded a space to the level of a vacuum, then when that space collapses with the full force of atmospheric pressure, there must be one large transient pressure drop in the region, then that creates a propagating wave outwards from that zone complete with flying glass. An implosion is only an implosion for a time, then the particles leave the area at a high velocity radiating outwards, analogous to an explosion.

Re: CRT implosion screens.
 

Yes, it showed all the signs of a severe explosion. The shadow mask used to be holed by lumps of funnel shooting through and if you were within 6 feet of the set you would be hit by glass for sure. Sometimes the shadow mask would fly out too and land on the floor. It was always the screen impact that did it. The nitrogen just made for a crazed area. One time I left it on for ages, see what would happen. Still nothing.

As my manager said after I said 'who is likely to swing a metal ball at it' that it represents anything from a child carrying something and falling against the screen to mishaps while transporting.

Re: CRT implosion screens.
 

I run a high vacuum system which consists of a 12" diameter 14" high borosilicate glass bell jar resting on a 1" thick steel plate. The glass is sealed to the plate by an "L" seal made of a special low vapour pressure rubber. The chamber is evacuated first by a fast roughing pump. That removes the majority of the air and gets down to 1 torr in a bout 20 minutes. After that the backing pump is started, then the high vacuum vapour diffusion pump when the pressure is less than about 50 milli-torr.

It is worth noting that the 1 torr "rough" vacuum is every bit as dangerous as the high vacuum. The difference between the force on the glass at high vacuum (1*10E-05 torr) and rough vacuum (1 torr) is miniscule.

If you do the sums, working out the area of glass and the pressure on every square inch it comes out at about 2 tons on the bell jar - say a large family car. I guess its about the same as a large CRT. I am protected from it by an acrylic safety cylinder - just in case. The most likely time for it to implode would be on pump-down. The bell jar looks like it was blown, and has some visible imperfections (bubbles). I've been using it for years now, but I never run it without the implosion shield.

Re: CRT implosion screens.
 

In the 1950's I remember dad getting rid of the CRT from our first TV by lobbing it out of the back door into the concreted part of our garden and rapidly shutting the door. A really impressive bang that had all the neighbours looking out, and tiny fragments of glass everywhere. Dad just expressed surprise like all the others! For the next one, he put it in a sack in the dustbin and lobbed bricks in until it went. Our retired drag-and-bone man neighbour said he just used to knock off the seal on the gun with a spade, which left the bulb intact.